1. Field of the Invention
The present invention relates to a display device comprising a display limit which has an array of capacitive elements arranged at respective picture elements and displaceable for turning on and off the corresponding picture elements, and a drive unit which has an array of drive circuits arranged at respective picture elements for driving the corresponding capacitive elements in response to input signals.
2. Description of the Related Art
The applicant of the present application has proposed a display device employing ceramic components as disclosed in Japanese laid-open patent publication No. 7-287176, for example. As shown in FIG. 22 of the accompanying drawings, the proposed display device has an array of actuators 200 associated with respective picture elements. Each of the actuators 200 has an actuator unit 208 comprising a piezoelectric/electrostrictive layer 202, an upper electrode 204 mounted on an upper surface of the piezoelectric/electrostrictive layer 202, and a lower electrode 206 mounted on a lower surface of the piezoelectric/electrostrictive layer 202, and a base body 214 comprising a vibrating section 210 disposed underneath the actuator unit 208 and a fixed section 212 joined to the vibrating section 210. The lower electrode 206 is held against the vibrating section 210, which supports the actuator unit 208 thereon.
The vibrating section 210 and the fixed section 212 are integrally formed of ceramics. The base body 214 has a recess 216 defined therein beneath the vibrating section 210 so that the vibrating section 210 is thinner than the fixed section 212.
A displacement transfer element 220 for providing a predetermined area of contact with an optical waveguide plate 218 is joined to the upper electrode 204. In FIG. 22, when the actuator 200 is in a normal state in which it is held at rest, the displacement transfer element 220 is positioned in the vicinity of the optical waveguide plate 218, and when the actuator 200 is in an energized state, the displacement transfer element 220 is brought into contact with the optical waveguide plate 218 by a distance equal to or smaller than the wavelength of light.
Light 222 is introduced into the optical waveguide plate 218 from a lateral end thereof, for example. The optical waveguide plate 218 has its refractive index pre-adjusted to cause all the light 222 to be totally reflected within the optical waveguide plate 218 without passing through front and rear surfaces thereof. When a voltage signal depending on the attributes of an image signal is selectively applied to the actuator 200 via the upper electrode 204 and the lower electrode 206 to hold the actuator 200 in the normal state or displace the actuator 200 in the energized state, the displacement transfer element 220 is controlled to move into or out of contact with optical waveguide plate 218. Thus, dispersed light (leaking light) 224 from a given area, aligned with the actuator 200, of the optimal waveguide plate 218 is controlled to display an image depending on the image signal on the optical waveguide plate 218.
The proposed display device is advantageous in that (1) the power consumption thereof can be reduced, (2) the illuminance of the display screen can be increased, and (3) when it is used in color display applications, it does not need to have more picture elements than black-and-white display device.
FIG. 23 of the accompanying drawings shows the above display device including peripheral circuits. The display device includes a display unit 230 having a matrix of picture elements, and the peripheral circuits include a vertical shift circuit 234 from which there extend as many vertical select lines 232 as the number of rows of picture elements, each of the vertical select lines 232 being shared by a number of picture elements (a group of picture elements) making up one row, and a horizontal shift circuit 238 from which there extend as many horizontal select lines 236 as the number of columns of picture elements, each of the horizontal select lines 236 being shared by a number of picture elements (a group of picture elements) making up one column.
Display information (output voltage) outputted from horizontal shift circuit 238 to a group of picture elements in a selected row is also applied to a group of picture elements in unselected rows, resulting in the driving of unnecessary picture elements (actuators). Therefore, the display device consumes an unwanted amount of electric energy, and is not suitable for low power consumption designs.
When all the rows are selected in a vertical scanning period, the display screen fails to produce images of high illuminance because the picture elements emit light only in a period of time represented by (vertical scanning period/required number of selected rows).
As shown in FIG. 24 of the accompanying drawings, one solution would be to use horizontal shift circuits 238 associated with the respective rows. The solution, however, is disadvantageous in that the resultant circuit arrangement would be very complex.
The applicant has proposed a new display device in order to solve the above problems (see the publication WO98/54609).
As shown in FIG. 25 of the accompanying drawings, the proposed display device, denoted by 300, has a switching thin film transistor (TFT) 308 disposed in the vicinity of an actuator 306 which comprises a lower electrode 302b, a shape holding layer 304, and an upper electrode 302a that are disposed on a drive unit.
The upper electrode 302a of the actuator 306 and a source/drain region 310 of the TFT 308 are electrically connected to each other by a contact 312. A select line 314 and a gate electrode of the TFT 308 are electrically connected to each other by a contact 316. A signal line 318 and a source/drain region 320 of the TFT 308 are electrically connected to each other by a contact 322.
With the above arrangement, it is possible to lower the power consumption, increase the illuminance, and simplify the formation of interconnections of the display device 300 which employs the actuator 306 including the shape holding layer 304.
The actuator 306 has a capacitor structure having a pair of electrodes which have a large capacitance. A 15-inch liquid crystal display unit having 1024×768 dots (XGA) has a square cell size of 0.295 mm on each side and an capacitance of 0.9 pF (dielectric constant εr=6.8, cell gap=6 μm). If the display device 300 has a 40-inch size and 1 g an XGA, then it has a square cell size of 0.8 mm on each side and an capacitance of 0.8 nF.
Since the display device 300 having the actuator 306 including the shape holding layer 304 has a larger capacitance than liquid crystal display units, it needs to be energized by a high voltage and a large current. If the TFT 308 is used as a switching element, then it suffers a withstand voltage problem. It is thus necessary to reduce the area of the actuator 306 per picture element to reduce the capacitance, but the aperture ratio of the picture element is reduced and the illuminance tends to be lowered.
If switching elements are constructed separately as an integrated circuit (IC), then a number of interconnections need to be provided between a drive circuit which has as many switching elements as the number of picture elements and a substrate on which actuators 306 are formed (actuator substrate). The proposal thus poses a new problem in that it is difficult to form interconnection patterns on the actuator substrate.